Photovoltaic cells, also called solar cells, are an important energy source that has become available in the past several years. Solar cells currently are being used in a number of commercial and consumer-oriented applications, including satellite and other space-related applications. Some solar cells have multi junction solar cell structures that include multiple sub-cells vertically stacked one above the other on a substrate. When solar cells in an array are receiving sunlight or are illuminated, each cell is forward biased. However, if one or more of the cells are not illuminated, for example because of shadowing or damage, the shadowed cells may become reversed biased in order to carry the current generated by the illuminated cells. Such reverse biasing can degrade the cells and can ultimately render the cells inoperable. To prevent reverse biasing, a diode structure is sometimes implemented. For example, U.S. Pat. No. 6,864,414, which is assigned to the assignee of the present application, discloses a monolithic multi junction solar cell structure with an integrated bypass diode formed above the top sub-cell layers. The purpose of the bypass diode is to draw the current away from the shadowed or damaged cell. The bypass diode becomes forward biased when the shadowed cell becomes reverse biased. Rather than forcing current through the shadowed cell, the diode draws the current away from the shadowed cell and maintains the connection to the next cell.
Although fabricating the bypass diode as an integrated device can enhance its efficiency and performance, it has been have discovered that fabrication of the integrated bypass diode sometimes reduces the open circuit voltage (Voc) of the top sub-cell, which can be undesirable.
The present invention is directed to improvements in solar cell structures.